Method of producing vanillin



1951 J. H. FESHER ET AL METHOD OF PRODUCING VANILLIN 3 SheetswSheet 1Filed Jan. '7 1949 1951 J. H. FISHER ET AL METHOD OF PRODUCING VANILLIN3 Sheets-Sheet 2 Filed Jan. '7. 1949 NOV. 27, 1951 J H, FISHER ET AL2,576,754

METHOD OF PRODUCING VANILLIN Filed Jan. '7. 1949 3 Sheets-$heet 5Patented Nov. 27, 1951 METHOD OF PRODUCING VANILLIN John Henry Fisherand Charles A. Sankey, St. Catharines, Ontario, Canada, assignors to TheOntario Paper Company Limited, Thorold, n-

tario, Canada Application January 7, 1949, Serial No. 69,817 In CanadaAugust 11, 1948 8 Claims.

This invention relates to the production of vanillin, acetovanillone andother oxidation products from lignosulfonic acid compounds such as wastesulphite liquor and especially from the same after treatment such thatthe fermentable sugar content thereof has been reduced.

It is well known that lignosulfonic acid compounds can be used toproduce vanillin and other products when subjected to oxidation undersuitable conditions in the presence of caustic soda or caustic potash.Cross reference is made to the co-pending applications of Marshall andSankey, Serial Number 606,690, filed July 23, 1945, now Patent No.2,516,827, and Serial Number 726,626, filed August 8, 1947, now PatentNo. 2,544,999, and to the two applications of Fisher and Marshall filedof even date hereof, S. N. 69,815 and S. N. 69,816, in which aredisclosed methods of effecting production of vanillin and co-productsfrom lignin-containing substances in a particularly advantageous manner.

In the co-pending applications of Fisher and Marshall filed of even datethereof, S. N. 69,815 and S. N. 69,816, methods are disclosed forproducing oxidation products including vanillin when lime is used as theactive alkali. Disclosure is also made that the yield of vanillin can beimproved by reduction of the initial concentration of lignin in thecharge to the batch reaction.

We have now discovered that a further substantial increase in vanillinyield may be obtained together with other beneficial effects hereinaftermentioned, when the oxidation reaction with lime as the active alkaliand in the presence of a finely dispersed gas containing free gaseousoxygen is conducted on a substantially continuous basis rather than on abatch basis and further that the increase in vanillin yield is greatlyin excess of that which can be predicted according to calculationscorrelating batch process and continuous process yields as per acceptedchemical engineering theory. We have also discovered that as applied tothe instant reaction optimum yields are obtainable with a single reactorunit operating on a continuous basis which use of a single unit is alsocontrary to accepted chemical engineering theory.

A comprehensive discussion of the factors correlating batch processyields and continuous process yields has been published by MacMullin andWeber, Trans. Am. Inst. Chem. Eng. 31, 409-458 (1935). When applied to aprocess, such as the instant one, which is characterized on a batchbasis by an increase in vanillin yield with reaction time up to a peakyield and by a decrease thereafter from this maximum yield, it will beapparent that the yield for a single batch reactor operating for a timecorresponding to the peak yield must necessarily be greater than theexpected yield for a continuous process in the same reactor under thesame conditions of temperature, pressure, concentration of reactants andagitation, since small increment portions of the reaction mixture insuch a continuous process will have been present in the reaction vesselfor all times from theoretically zero to theoretically the full time forwhich the reactor has been in operation from its start-up. Suchincrement portions will, as discharged from the reactor, correspond toall times over the whole range of the time-yield relationship as appliedto the particular reaction in the particular reaction vessel.

The theoretically expected yield from a continuous process may thereforebe calculated, provided that the time versus yield relation for batchoperation is known, by a consideration of the distribution of holdingtimes in the reactor, which distribution is a probability functionderived in the publication of MacMullin and Weber aforesaid.

When a continuousprocess of the character described is carried out in agroup of reactors in series-the continuous process yield is also afunction of the number of such reactors. Theoretically a large number ofreactors is required to approach the batch process yield and thisfunctional relationship is also discussed in detail by MacMullin andWeber. In actual chemical engineering practice the cost of'installingand using a number of reactors in series in a continuous processoperation must be balanced against the increase in yield which resultsfrom the use of a large number of such reactors.

As applied to the instant reaction the use of a series of reactors forthe continuous process would be justified according to accepted chemicalengineering theory. Contrary to this theory we have discovered that themaximum yield of vanillin for a continuous process is obtained when oneand only one reactor is used.

In verification of the foregoing it is instructive to apply the methodsof MacMullin and Weber to instant reaction. There is shown in Fig. 2 therelationship between the yield of vanillin and the time of reaction in abatch process as experimentally determined by use by spectrometricanalyses as hereinafter discussed under total reactants having areaction timein the range of t to (t-i-dt) i. e.

dzc=f(t, T,jn

Let y=the batch yield at time t and Y=the theoretical continuous processyield. The latter yield, Y, can be calculated by summation of all thedifferential products yda: through values of t from zero to infinity, i.e.

Y= we to Values of y'at any time t may be read from Fig. 2. Valuesbf dxhave been calculated in the form of probability curves by MacMullin andWeber.

Fig. 3 is reproduced from the latter reference and-supplies pertinentdata for the calculations below. In this Fig. '3 the curves numbered 1,2, 3, 5, 10, '20 refer respectively to 1, 2, 3, 5, 10 and 20 reactorsin'series in continuous flow. Curve 1 is applicable to the presentconsideration of a single reactor. These data assume conditions ofperfect mixing in the continuous reaction system and thereforecorrespond to the ideal case of the instant application when operated ona continuous basis.

The integral the reactants which has a holding time in the range 30 to40 minutes in a single continuously operated reactor for which theaverage holding time is 100 minutes.

From Fig. 2 the batchyield corresponding to 35 minutes reaction time is3.1%.

From MacMullin and Webers equation as plotted in curve 1 of Fig. 3 thevalue of A2: at 35 min. is 0.070.

The product y.A:z: is therefore (Note-The value 0.070 is obtained fromFig. 3 as follows:

In this figure y is plotted against where dy is equivalent to da: asused in the instant specification 4 0 is the average holding time forone reactor 1t is the number of reactors Converting to the symbols usedin this specification dx y :1" dt since hence- .n IA: T For a ratio t 35min. "T 1111117 'y'=0.70 as per curve 1 of Fig. 3

Therefore Thenegative sign for A0: is because the slope of .MacMullinand Webers curve is decreasing.)

A summation bylO minute time increments is given in Table v.1.Increments are included up to .a ratio The appropriate values of .Am:were obtained by extending the plot ofcurve 10f Fig. 3 using Mac-Mullin and Webers equation y" e T TABLE 1 'Time, min. g; Ax yvPer cm 0.0 4 0.0952 0. 057 1. 3 0. 0860 0. 112 2. 2 0. 0778 0. 171 3. 1 0. 0704 10. 217 4. 0 0. .0033 0. 253 4. 1 0. 0573 0. 237 4. 2 0. 0323 0. 220 4. 30. 047,2 0. 203 4. 4 0. 0427 0.133 4. 4 0. 0335- 0. 109 4. 4 0. 0345 0.152 4.3 0. 0313 0. 4. 3 0. 0235 0. 123 4. 3 0. 0257. 0. 111 4. 2 0.02320. 097 4. 1 0. 0211 0. 087 4. .1 0. 0190 0. 073 4. 0 0. 0170 0. 070 3. 90. 0158 0.052 3. 9 0. 0140 0. 0'5 3. s 0. 0129 0. 049 a. 7 0. 0113 0.042 3. 0 0. 0101 0. 030 3.5 0. 0090 0. 032 3. 4 0. 0032 0. 02s 3. 2 0.0078 y 0. 025 3. 1 0. 0009 0. 021 3. 0 0. 0052 0. 019 2. 9 0. 0057 0.017 2. 7 0. 0051 0. 014

The above calculations establish that the expected continuous'processreaction yield amounts to 3.08% neglecting theefie'ct-of material havinga holding time greater than 300 min. It is apparent that the producty.Aa: becomes negligibly-small after 300-min. and thata yield'of 3.1% isthe expectedcontinuous process yield for all practical purposes.

As the peak batch yield is 4.4%, the above calculation is ademonstration that the expected yield from a continuous reaction systemfor one reactor with an average holding time of 100 minutes is only3.1+4.4 or 70.4% of the peak batch yield.

As noted previously the yield curve data for Fig. 2 were obtained byspectrometric analyses.

The absolute accuracy of the value of the yield is unimportant to theabove correlation of batch and continuous process yields as lon as thegeneral shape of the curve is valid, and the analytical data arecomparable.

By calculations analogous to the above, the expected continuous processyield for a single reactor is 3.15% for an average holding time of 125minutes and 3.05% for an average holding time of 150 minutes. Suchcalculations may be made for any given average holding time and thiswill establish that for a continuous reaction system with a singlereactor no average holding time may be selected for the instant reactionunder the instant reaction conditions for which the continuous processyield will substantially exceed 71% of the peak batch process used.

Similar calculations may be made from systems of 2, 3 and indeed anynumber of reactors operated in series under conditions of continuousoperation. In every case and for every holding time the expectedcontinuous process yield is less than the peak batch yield, it beingunderstood of course, that the expected continuous process yieldapproaches the peak batch yield as the number of reactors is increased.

In spite of these facts we have concluded that, owing to the dilutioneffect as disclosed by Fisher and Marshall in their co-pendingapplication filed of even date hereof S. N. 69,816, for this particularreaction the use of a continuous process would produce a novel resultand that the calculations as per MacMullin and Weber were inadequate.The continuous introduction of lignosulfonic acid compound into thereaction vessel containing a lower concentration of the samelignosulfonic acid compound (because a portion of such initialconcentration has already been used in the course of the oxidationreaction) would have the overall effect of causing the reaction to takeplace at a lower concentration of lignin than in the case of a batchprocess. The demonstration of the correctness of these conclusions willappear in the examples hereinafter furnished.

We have also discovered, and it is a further important advantageresulting from our invention, that the settling characteristics of thesludge residual from the reaction are greatly improved, therebypermitting easier separation of the liquid and sludge portions and themore readily recovery oi the valuable compounds including vanillin andacetovanillone dissolved in the liquid portion. vanillin and co-productsfrom such liquid portion has already been disclosed in the applicationsof Fisher and Marshall aforesaid.

The selection of preferred conditions and the necessarily functionalcharacter of this selec-'- tion for the instant reaction has beendisclosed in the co-pending applications of Fisher and Marshall filed ofeven date hereof S. N. 69,815 and S. N. 69,816. As applied to continuousprocess operation the disclosures of Fisher and Marshall are fullypertinent to this point. We have verified that for continuous processoperation preferred conditions of temperature and partial- The knowngreater ease of recovery of oxygen pressure lie within the rangedisclosed by Fisher and Marshall, namely, temperatures.

in the range C. to 200 C. and partial oxy-' gen pressure less than 20lb. per square inch,

than 12 ,nor greater than that inherent in the use of lime as the activealkali. In the continuous process a reduced yield due to insufi'icientactive alkali may be restored by the addition ofmore lime or sometimesby the reduction of air flow or of the reaction time, the latter twochanges tending to reduce the rate of produc tion of acidic materials.We have observed that when conditions of insufiicient alkali exist thereaction discharge usually becomes much darker in colour. We have alsoverified that for continuous processing as well as for batch pro-cessinga reduced concentration of lignin in the range 10 to '70 grams ligninper litre in the materials entering the reactor vessel results inimprovement in yield of vanillin on a lignin base, as disclosed inco-pending application of Fisher and Marshall, S. N. 69,816.

In practicing our invention we employ apparatus as-illustrated intheaccompanying drawings, in which Fig. 1 represents in diagrammaticform the equipment used by us for producing the oxidation productsherein discussed.

Referring to Fig. 1, we employ a reaction vessel l equipped with a cover2 and with provision for agitation, the type indicated being theturbomixer 3 driven by the motor 4. The lignosulionic acid compound, forexample alcohol plant effluent, is introduced from its storage tank 5through the proportionating pump 5 via pipeline 1 into the reactor. Theslaked lime slurry is in-- troduced from itsstorage tank 8 through theproportionating pump 6 which regulates the relative volumes of thereactants and through the pipeline 9 into the reactor l. The level inthe reactor is controlled through the auxiliary level control tank [0and the level control mechanism l I which operates the discharge valvei2, the reactor discharge going out through the pipeline l3 controlledby the valve 12 and through the cooler I4. The pipelines i5 and itprovide respectively the liquor phase and gaseous phase connectionsbetween the reaction vessel l and the level control tank l0. Air isintroduced from the compressor I! through the tank i8 and valves I9, 20which control the pressure and rate of air flow, the air beingintroduced into the reactor through the pipe 2| and dispersed therein bythe turbo-mixer 3.

Heat is provided by steam produced in the boiler 22 regulated as topressure and volume awna usedzwhen requ re is.v Erovis nisimadeu pntema.

- neraturemeasu-rement the...-r asto zhr t e:

n; this. sp c fication; thea enin. content; of. a

li nimccntain na. sub ance s. m s d t rms it emeth yl. cont nt reo ur.xpe i e tsjt, is. sumed; hat. her methoxyl content to lignin content ofany; of;- the; mixtures investigated,isv 15.5..- to'; 109.1 Such an,assulllfid. .ratiois in accordance. withwcur ent, good chemical; usageindealing; with. ligpin containing substances. Whether, this. assumedratiois or is not numerically correct is. inn-1.12;; al because he.ratio methc Y-J to; enin may; be; reasonably assumed to be. constantfor, an iven gn mcp n as s a e The-ass sumption of the above ratio will,therefore. servefor pprposes of obtaininga, valid relativeuide o.hequant ty f li ninfindisninnr" tainingsubstances. 7

In; analyzing .materials for;.theinvanillincon--. tent two general.techniques have. been-em:- ployed. The. first isa gravimetric; procedureinvolving the separation and estimation oi vanil-; linin the form. ofits m-nitrobenzoyl. hydrazone. This, is. described by; Buck-land.Tomlinson. and- Hibbert, Qan. J .Research 1.63 54. (1938.), and is the;moreaccurate; of the two procedures -but. involves a tedious analyticaloperation... .We ;em-. ploy ether as. a solvent in place,- oftrichloroethylene as described in the aforementioned referenc.e. Thesecond is a spectrometric methqdv of analysis according to the general.

method of Lemon,- Ind. Eng. Chem, Anal. Ed. This is a rapid.procedure-Which gives anoverall measure of the:,substancespres:. entwhich are spectrometrically active. at .ap proximately; 3500 the.vanillin. bein .i theprincipal substance present which'isso-active. Thismuch more simple andrapid. procedure-has been used-in a great deal ofourworkas arela.-. tive guide to the vanillin .content ofthewariousresidues and has; been frequently applied with. the: use. of anappropriate empirical correction.- batch;1naterial. shouldremaininthereactorat- 19;),minutes. The above result istheref ore 99%.;representative. of the; performance; under con--- ous op ation f r. polo ed periods .of. .0%

factor based on theratioof the gravimetricto; spectrometric resultswhere both of, these. have been determined. for the. same; type. ofresidue.

The. following descriptionof experiments.v whichv have been performedvby uswill, serve. to illustrate the application and practiceoiour in-..vention. It is to be .understood that-.our inven.- tion is not limitedto the vmaterials; and. condi: tions: described in theseexperiments-which are, to, be considered. as examples only.

waszsubieeli dt t a batchi nesreac ienzmdsr th followin conditions;

ime fo ba h. to. reach 1.7. y d rect;steam-,..

ing-2.01 V Time, as batch, at 170"C.'1 hr.

Air-.flovv32 lb. per. hr, Reactor ,pressure-,155 lb". gauge Partiarpresure i. n-condens lejeasesb. m (L rrespcmding partia o y n. p e. e s thanwith 1 feed rates Alcohol plant efiluentfllflia -U.

per-hi2,

Reactor volume was maintained': at: 3.9.6.; U; 8: gal.bythe:levelcontrolequipment;

Observed discharge. rate from reactor.-30J. U. S. gal. per hr. (thisisqgreater than thecom bined -feed rates of reactants because. directsteaming vzasused for heating.

Averageholding time in. thereactor: I

- hi== .7 9 m ut s.

Concentration; of. li in. to: e: reacto by analysis of alcohol ramslperzlit g.

U... 8.. al; f. r actor discharge-1w recollec e A. sample i. thereactdischar e wa he aken. andshowed. y -ra i analy is.

vanillin yield of 8. n a. l in b si It: may be, calculated by themethods .of--. Mac;

Mul in. and. W be t. 0%, 0f. h sina reprcsentativepf the originalvbatchmaterial.

results obtained for the group of experiments are 55 given in Table 2;.

Example 1 A charge of- 16.4110. of .slakedlime, 13.9; U. 8..

gal; alcohol. plant. eflluent and. .1 U. S. gal.-

water was pumped intoAthe reaction-vessel which In, this; tablez.

Col; (l) isan identifying code number refere ring to. our records ofexperiments.

C0142), isdetermined by analysis of the alcohol.

had'been preheatedwith steam to 1.7.0" 0.. nlanteiiluent andthe.known-dilutioniactors.I

S. gal. per hr. lime slurry 'of. 1.02: lb. slaked lime-per-U; S; gal.of'waten ataarate oi 12.4 U. S; gal. slurry-.-

plant t luentand lr et i 1 materials. dilutin t is; orr sponded: to;

Col. (3) is calculated from the known volume of charge in the reactorand the observed rate of discharge.

Col. (4) is the vanillin yield on a lignin basis, all analyses by thegravimetric method, and based on samples after a continuous reactionperiod of 190 min.

Col. (5) is the peak batch yield, gravimetric analyses, as observed forthe same alcohol plant efliuent and disclosed byFisher and Marshall intheir co-pending application S. N. 69,81 6, filed of even date for thesame lignin concentration.

Col. (6) is calculated by taking 71% of col. (5). This correlationhasbeen previously established for the conditions of cook 193. As appliedto cook 227 the application of the same ratio is a reasonablesimplifying assumption because the general shape of the yield curvewithtime is the same for cooks of the various dilutions.

Col. ('7) expresses the obtained continuous process yield as apercentage of the expected continuous process yield.

Example 2 The yield from a system of two reactors was determined for thesame alcohol plant effluent using the same apparatus by operating thesingle reactor under conditions to give one half the average holdingtime (i. e. 50 min.) and then rerunning the partially processeddischarge a second time under the same conditions, so that the totalaverage holding time was 100 min. The conditions of temperature,pressure, air flow and charge volume were as per Example 1. The ligninconcentration corresponded to 35 grams per litre. The vanillin yield asdetermined by spectrometric analysis was 5.5%. By correlation with thedata of Table 2 it will be apparent that the yield has not been improvedby a procedure equivalent to the use of two reactors instead of one.

Example 3 The relative settling rates of the sludges produced may beestimated by measuring the ratio of the volume occupied by the sludge tothe total sample volume, under comparable conditions of concentrationand settling time. Using a cylindrical settling bottle, '7" high, and asettling time of 16 hours, we have observed thesettled sludge volume tobe normally in the range 0.28 to 0.32 for continuous processed alcoholplant eflluent with a corresponding lignin concentration to reactor of38 g. per litre. By comparison the settled sludge volume of a batchprocess sludge processed at the same lignin concentration to reactor was0.55.

When in this specification we use the expression lignosulfonic acidcompounds we mean thereby materials derived from lignin when so treatedthat sulfonic acids are formed therefrom, e. g. when lignin-containingsubstances are subjected to the sulphite pulping process, and includingsalts of the said sulfonic acids.

When in the claims we refer to a pH of 12 or greater, we have referenceto the pH of a sample withdrawn from the reactor after being cooled toroom temperature. In the determination of this pH we have used a glasselectrode especially designed to be accurate in the range of highalkalinity measurements.

What we claim as our invention is:

1. A method of producing oxidation products including the calciumderivative of vanillin from lignosulfonic acid compounds in an alkalineaqueous medium containing lime as the active 10 alkaliin an amountsufficient to maintain the pH of the reaction mixture, determined on asample withdrawn from the reaction zone and cooled to room temperaturein the range not less than 12 to not more than that of a saturatedsolution of lime in the same mixture, which comprises introducing saidcompounds into a reaction zone, heating the reaction mixture to atemperature not less than 120 0., nor more than 200 C'., maintainingsaid zone under superatmospheric pressure, continuously passing a gascontaining free oxygen in finely dispersed form through said reactionzone, continuously removing residual gas, the rate of addition andremoval of said gas being such as to maintain a partial pressure ofoxygen in said zone of less than lb. per square inch, the time of saidreaction being less than four hours, the reactants being continuouslyintroduced into, and the reaction products being continuously removedfrom, the agitated reaction mixture.

2. A method of producing, in a single reactor, oxidation productsincluding the calcium derivative of vanillin from lignosulfonic acidcompounds in an alkaline aqueous medium containing lime as the activealkali in an amount sufficient to maintain the pH of the reactionmixture determined on a sample withdrawn from the reaction zone andcooled to room temperature in the range not less than 12 to not morethan that of a saturated solution of lime in the same mixture, whichcomprises introducing said compounds into a reaction zone, heating thereaction mixture to a temperature not less than 0., nor more than 2000., maintaining said zone under superatmospheric pressure, continuouslypassing a gas containing free oxygen in finely dispersed form throughsaid reaction zone, continuously removing residual gas, the rate ofaddition and removal of said gas being such as to be maintain a partialpressure of oxygen in said zone of less than 20 lb. per square inch, thetime of said reaction being less than four hours, the reactants beingcontinuously introduced into, and the reaction products beingcontinuously removed from, the agitated reaction zone in the saidreactor.

3. A method of producing oxidation products including the calciumderivative of vanillin from lignosulfonic acid compounds, the initiallignin concentration of such compounds in an alkaline aqueous medium,measured as herein prescribed, being not more than 70 grams per litrenor less than 10 grams per litre, the said medium containing lime as theactive alkali in an amount sufiicient to maintain the pH of the reactionmixture determined on a sample Withdrawn from the reaction zone andcooled to room temperature in the range not less than 12 to not morethan that of a saturated solution of lime in the same mixture, whichcomprises introducing said compounds into a reaction zone, heating thereaction mixture to a temperature not less than 120 C., nor more than200 C., maintaining said zone under superatmospheric pressure,continuously passing a gas containing free oxygen in finely dispersedform through said reaction zone, continuously removing residual gas, therate of addition and removal of said gas being such as to maintain apartial pressure of oxygen in said zone of less than 20 lb. per squareinch, the time of said reaction being less than four hours, thereactants being continuously introduced into, and the reaction productsbeing continuously removed irom, the agitated reaction mixture.

r-perlitre, the said medium containing lime asthe active' alkali in anamount sufiicient to main- "tain'the pH of the reactionmixturadeterniined on' a sample withdrawn from the reaction zone andcooled to room temperature in the range'not less than 12 tonot more thanthat of a saturated :solution'of lime in the same'mixture,' whichcomprises introducing said compounds 7 into: a reaction-zone,heating'the 'reaction mixture to a temperature not'less than 120 0., normore than 200'-C., maintaining said zone under-superatmosphericpressure, continuously passing a gas containing free oxygen in finelydispersed form through said reaction zone, continuously're'movingresidual gas,the'rate of addition'and're- 'moval of said gas being suchas to 'gmaintain a martial pressure of oxygen in said zone of less than20 lb. per square inch,'the time of said re-r action being lessth'anfour-hours, the reactants being continuously introduced into-andthe'reaction products being continuouslyremoved from, the agitatedreaction zone in "the said reactor.

in an alkaline aqueous niediumy 12 The process of *clainr 1 "wherein thelignosulfonic a'cid compound is 'sulphite waste liquor.

6. The process of claim 1 wherein -the- 1igno- :sulfonic a'cid compoundis sulphite waste liquor 'whichh'as been-previously treated toire'ducethe fermentable sugar 'conten't'thereof.

7. The process of claim 3, further 'character- :izedin'that theconcentration of lignin isnot more'than" about :38 fgrams. per liter.

'8. The; process of 3 claim 4, further 'characteriz'd in'that" theconcentration of lignin is not more-than about '38 :grams per liter.

- .'-JOHN HENRY FISHER.

'I-CI-I'A'RDES A. =SANKEY. REFERENCES' GITED Tlie folldwing"fefrencesare of record in the file of this pate'nt:

'UNITED "STATES PATENTS Number Name Date 2,099,014 Hatch Nov. 16, 1937OTHER REFERENCES ,er; No.5318,386jEreudenberg et"- al. '(A. P. c.)

1. A METHOD OF PRODUCING OXIDIZATION PRODUCTS INCLUDING THE CALCIUMDERIVATIVE OF VANILLIN FROM LIGNOSULFONIC ACID COMPOUNDS IN AN ALKALINEAQUEOUS MEDIUM CONTAINING LIME AS THE ACTIVE ALKALI IN AN AMOUNTSUFFICIENT TO MAINTAIN THE PH OF THE REACTION MIXTURE, DETERMINED ON ASAMPLE WITHDRAWN FROM THE REACTION ZONE AND COOLED TO ROOM TEMPERATUREIN THE RANGE NOT LESS THAN 12 TO NOT MORE THAN THAT OF A SATURATEDSOLUTION OF LIME IN THE SAME MIXTURE, WHICH COMPRISES INTRODUCING SAIDCOMPOUNDS INTO A REACTION ZONE, HEATING THE REACTION MIXTURE TO ATEMPERATURE NOT LESS THAN 120* C., NOR MORE THAN 200* C., MAINTAININGSAID ZONE UNDER SUPERATMOSPHERIC PRESSURE, CONTINUOUSLY PASSING A GASCONTAINING FREE OXYGEN IN FINELY DISPERSED FORM THROUGH SAID REACTIONZONE, CONTINUOUSLY PASSING A GAS CONTAINING GAS, THE RATE OF ADDITIONAND REMOVAL RESIDUAL BEING SUCH AS TO MAINTAIN A PARTIAL PRESSURE OFOXYGEN IN SAID ZONE OF LESS THAN 2/ LB. PER SQUARE INCH, THE TIME OFSAID REACTION BEING LESS THAN FOUR HOURS, THE REACTANTS BEINGCONTINUOUSLY INTRODUCED INTO, AND THE REACTANT PRODUCTS BEINGCONTINUOUSLY REMOVED FROM, THE AGITATED REACTION MIXTURE.